1. Field of the Invention
The present invention relates to a piezoelectric ceramic, a method of producing the same, and a piezoelectric part and more particularly to a technique by which the piezoelectric properties of a piezoelectric ceramic can be enhanced and the dispersion of the piezoelectric properties can be reduced.
2. Description of the Related Art
Referring to a technique interesting to the present invention, Japanese Unexamined Patent Application Publication No. 4-295051 (Patent Document 1) describes a PZT type ceramic composition in which bismuth sodium titanate is added to a PZT type material represented by the following chemical formula: Pb1.01(Zr0.53Ti0.47)O3+0.5 mol % Nb2O5 in an amount of not less than 0.5% by weight to less than 5% by weight based on 100% by weight of the PZT type material.
The known technique described in Patent Document 1 has the object of enhancing the mechanical strength of a piezoelectric ceramic without the functional characteristics of the piezoelectric ceramic being damaged. To achieve the above-described object, according to the known technique, a bismuth sodium titanate with a piezoelectric property is selected as an additive which can form a liquid phase at the same temperature as the firing temperature of the PZT type material and also has the same functional properties as those of the PZT type material. The additive is added, and forms a liquid phase when the PZT type material is fired. Thus, the PZT type material is sintered in a liquid phase.
Referring to another technique interesting to the present invention, Japanese Unexamined Patent Application Publication No. 11-87792 (Patent Document 2) describes a piezoelectric ceramic having a composite structure, in which at least two regions having different compositional ratios or different compositions exist. One of the two regions is a region in which the properties of the piezoelectric ceramic are prominent, and the other is a region in which the properties of an electrostrictive ceramic are prominent.
The known technique described in Patent Document 2 has the object of providing a piezoelectric ceramic of which the electric field-strain characteristic is controlled so as to be linear. To achieve this object, according to the known technique, materials with different piezoelectric properties are processed to form a composite. In particular, one of the materials forms a region in which the properties of a piezoelectric ceramic are prominent, and the other material forms a region in which the properties of an electrostrictive ceramic are prominent. Thereby, the piezoelectric characteristics can be easily controlled. As a result, the electric field-strain characteristic can be controlled.
However, the techniques described in Patent Documents 1 and 2 have the following problems.
According to the technique of Patent Document 1, the PZT type material to which bismuth sodium titanate is added is sintered in the liquid phase. Therefore, the PZT type material can be sintered at low temperature. However, the properties of the sintered piece may be dispersed, since the sintering is not uniform in the case of sintering in a liquid phase. Moreover, since bismuth sodium titanate has a lower dielectric constant than the PZT type material, a voltage can not be sufficiently applied to the sintered PZT type material in some cases, when the PZT type material is poled. Thus, regarding the poled piezoelectric piece, the piezoelectric properties can not be sufficiently realized in some cases.
According to the technique described in Patent Document 2, two-type regions are formed. In particular, two types of powdery materials with different composition ratios or compositions are respectively granulated. The granulated particles are mixed, formed and fired. However, when the granulated particles with different composition ratios or compositions are mixed and fired, the sintering tends to become irregular. Therefore, the piezoelectric properties are deteriorated or dispersed to a large degree in some cases.